1. Field of the Invention
The present invention generally relates to a packet transmission method and-system, more particularly to the packet transmission method and system for dividing a normal transmission packet into a plurality of data units each having a shorter data length, and scheduling transmission order of those data units.
2. Description of the Related Art
Today, on the Internet, services have been provided that can support various quality of service (QoS). QoS may be defined by allowable packet loss, allowable delay time, and the like.
For example, between transmission of E-mail and transmission of audio or video streaming, there is a big difference in the required QoS. In data type transmission such as for E-mail or Web pages, it may be more important to ensure an arrival of all the information (reliability or low bit error) than an arrival with no or minimal delay. That is, the data type transmission may allow some delay if necessary for the reliability. While, in real-time type transmission such as for audio or video streaming, minimal delay is more important because the real-time type data can be practically unusable if the delay is too large.
Some architectures offer different QoS such as Intserv (Integrated Service) and Diffserv (Differentiated Service). The Intserv guarantees throughput for every QoS class, which is a so-called bandwidth-guaranteed type service, and utilizes RSVP (Resource Reservation Protocol). The Diffserv sets up a priority for every packet, based on user (IP address) or contents of transmitted information.
A conventional packet transmission method and system is described below with reference to FIG.1. FIG.1 schematically shows the configuration of the conventional packet transmitting apparatus.
In FIG. 1, a classifying part 101 processes packets (IP packets) to be transmitted to classify them on the basis of QoS requirement. It is assumed as an example that roughly there are some service classes classified as real-time type and some service classes classified as data type.
Alternatively, for classifying, a different service class is assigned to each flow if this node is an RSVP router, while service classes are allocated based on DSCP (Diffserv Codepoint) in IP header if the node is a Diffserv router.
Each classified transmission packet is managed in an IP datagram queue, using various buffer management techniques for each queue. As an example, some packets are dropped according to a predetermined discard policy such as RED (Random Early Detection) or RIO (RED with In/Out). The IP datagram queues are provided for each service class in an IP queue 102.
A transmission order for the packets to be transmitted buffered in each queue is scheduled by a scheduling part 103 according to a scheduling technique such as PQ (Priority Queuing), WFQ (Weighted Fair Queuing), or CBQ (Class-based Queuing). The scheduling may be based on allocated bands in case of the above-mentioned Intserv, while it may be based on the priority of each queue in case of the above Diffserv. Anyway, a front packet in each queue is pulled out according to one of the above techniques, and is transferred into a data link layer (the second layer).
The packets to be transmitted transferred into the data link layer are transmitted by a data link control part 104.
As described above, the conventional packet transmitting apparatus performs the QoS control by scheduling the packets to be transmitted in the IP layer on the basis of the QoS class.
However, the above conventional QoS control assumes the use of wired transmission where fine connections are maintained, and performs the same process to each QoS class in the data link layer (the second layer).
If such QoS control method is applied to radio transmission where packet losses may likely occur in radio paths, a larger delay may adversely occur in the real-time type packets requiring minimal delay. It is described as below.
With reference to FIG. 2, the configuration of a typical mobile communication system is now described. FIG. 2 shows schematically the whole configuration of the typical mobile communication system.
In FIG. 2, a plurality of radio network controllers (RNC) 202 are connected to a core network (CN) 201, and each radio network controller 202 controls a plurality of base stations (BS) 203. Each radio base station 203 manages a plurality of cells 204. The number of each kind of stations shown is only an example and may include others.
In the mobile communication system, the data link control is usually performed in the RNC 202.
In the mobile communication system having such configuration, when the packet loss occurs, the same packet is usually retransmitted in the radio path in order to maintain the quality of transmission. With a typical control, when one packet is lost in the radio path, a receiver-side awaits the retransmission of the lost packet even if the subsequent packet is successfully transmitted.
As mentioned above, since reliability (low bit error) is important in the transmission of the data type packets, a process required for the retransmission control is applied to each of the packets to be transmitted in the data link control.
However, applying the data link control including the retransmission control uniformly to all the packets belonging to all QoS classes, consequently the retransmission control is applied to the real-time type packets.
In the radio transmission, particularly the retransmission control is frequently performed. Therefore, applying the above conventional QoS control to the radio transmission, the transmission delay may likely occur in the transmission of the real-time type packets where minimal delay may be more important than the reliability.
Also, transmission rate in the radio transmission is slower than one in the wired transmission, and hence longer packet transmission time is required in the former than in the latter. For example, when a packet having 1,500 bytes is transmitted at 128 kilobit per second (kbps), it takes approximately 100 milliseconds (ms). In audio or voice communication, 100 ms delay is quite large.
Therefore, when a real-time type packet is to be transmitted during the transmission of the data type packet, according to the above example, even if the scheduling process is performed taking account of QoS (i.e. the real-time type packet shall be given higher priority for transmission), this real-time type packet waits to be transmitted at most for 100 ms.
Thus, in the conventional data link control apparatus, the QoS control is exclusively performed in the IP layer (the third layer), and the QoS is not adequately reflected in the process of the data link layer (the second layer).
Therefore, the conventional packet transmitting apparatus has a problem that the transmission delay is likely to occur in the real-time type packets when it is used for the radio transmission.
Japanese laid-open patent application No. 2000-224261 discloses a data link control method in which a plurality of QoS planes are generated in the data link layer on the basis of the QoS information of the IP layer.
In this method, in fact, the data link control varies according to the QoS. However, although this method varies its retransmission method on the basis of the QoS requirement, the above-mentioned problem that the delay may occur in the real-time type packets due to the occurrence of the retransmission packet waiting time remains, because this method does not include a mode that does not perform the retransmission method.
Also, in this method, although a frame length to divide into is varied in the data link layer according to conditions of the radio transmission path, a frame by frame scheduling is not performed. Therefore, this method does not solve the problem that delay may occur in the real-time type packets because of the transmission waiting time until completion of transmission of the data type packets.